Kurt,
Go out and get a cheap "hot" knife if you are going to cut alot of foam. No pieces little whatsoever and the heat seals the foam a little. I use alot of foam in making GFRC monuments. Also, if you are going to make some high cubic sizes you can leave a void inside your block if you plan on sealing the luan with glass.
The air pocket would be more buouyant than the foam.